Which rulebook do junior and club umpires use, and can competitions vary it?

Netball Australia uses World Netball's Rules of Netball, 2024 Edition, which took effect in Australia on 1 January 2024. It applies to international matches and affiliated competitions and events. The PDF is available from the Netball Australia rules page: https://netball.com.au/rules-of-netball

Rule 21 allows variations for other levels of play. The rules are written for international play, and the controlling authority for a competition normally decides any variations. For primary school age children, more extensive modifications may be desirable, such as a lower goalpost or a smaller ball. Examples of variations the rulebook lists include:

• time played in halves (up to 20 minutes each) or in shorter quarters, and shorter intervals;
• a lower grade (2-ply) ball;
• mixed or same-gender teams;
• no reserve umpire, and fewer team officials (the primary care person must not be a player);
• fewer technical officials, but at least one scorer and one timekeeper.

Source: https://netball.com.au/rules-of-netball and Rules of Netball 2024, Rule 21.